The invention relates to a device for optically measuring the height of a surface of an object, which device comprises a radiation source for generating an irradiating beam which is directed along a predetermined beam axis and forms an irradiated spot on the surface, and an imaging system for converging radiation from the irradiated spot to a radiation beam forming an image spot on the beam axis in the image space of the image system.
Such a device is used, for example, to check whether a surface has the desired profile or whether a flat plate is actually fiat and does not have any grooves or projections. The device may also be used to check whether holes and elevations are provided at the correct positions on a substrate plate and may further be used, for example, for inspecting electronic circuits comprising electrically insulating plates provided with conducting metal strips and electronic components. Another use of the device is focus-point detection.
A device as described in the opening paragraph is known, inter alia, from the article "Optical Surface Microtopography Measurement and/or Automatic Focusing" published in IBM Technical Disclosure Bulletin, Vol. 15, No. 2, July 1972, pages 504-505. In this device a laser beam is focused by a lens system to form an irradiated spot on the surface of an object. Part of the radiation reflected by the surface is captured by the lens system and subsequently reflected out of the laser beam by a beam splitter. A further beam splitter forms two sub-beams from the reflected radiation beam. Each sub-beam comes to a focus, forming an image of the irradiated spot. The radiation in each sub-beam is measured by a detector with a pin hole aperture on the axis of the sub-beam and in front of the radiation-sensitive surface of the detector. In one sub-beam the pin hole is placed in the sub-beam before the image formed by the sub-beam, in the other sub-beam the pin hole is placed in the sub-beam behind the image formed by the other sub-beam. The output signals of the two detectors are fed to a differential amplifier. When the output signal of the differential amplifier is zero, both detectors receive an equal amount of radiation. Consequently, the two images must be equidistant to the respective pin holes. In other words, the focus or position of highest intensity of the image spot is midway between the two pin holes. A change in the height of the surface, causing a change in the position of the irradiated spot, leads to a change in the position of the image spots. The pin hole closest to the displaced corresponding image spot will capture more radiation than the pin hole further away from its image spot, which results in different output signals of the two detectors and in an output signal of the differential amplifier unequal to zero. The output signal of the differential amplifier is thus a measure of the height of the surface.
The range over which the known device can measure the height is determined by the axial distance between the two pin holes. A large distance between the two detectors yields a large height range, a small distance yields a small range. The resolution of the height measurement is inversely proportional to the height range. This means that a device with a small height range can measure the height with a relatively high resolution, whereas a device with a large height range can measure the height with a low resolution. A drawback of the known device is that it cannot measure heights over a large range with a high resolution.